Gas concentration measuring apparatus

ABSTRACT

A gas concentration measuring apparatus is provided which works to calculate the concentration of a given gas component with enhanced accuracy. The gas concentration measuring apparatus 1 includes, a gas sensor 10 and a calculating portion 11. The gas sensor 10 is equipped with a pump cell 3, a monitor cell 4, and a sensor cell 5. The calculating portion 11 subtracts a monitor cell current Im that is a current flowing through the monitor cell 4 from a sensor cell current Is that is a current flowing through the sensor cell 5 to calculate the concentration of the given gas component in gas g. When calculating the concentration of the given gas component, the calculating portion 11 performs a correction operation to bring a value of the monitor cell current Im close to a value of an oxygen dependent current Iso that is a component of the sensor cell current Is which arises from the concentration of oxygen.

This application is U.S. national phase of International Application No.PCT/JP2014/072719 filed 29 Aug. 2014 which designated the U.S. andclaims priority to JP Patent Application No. 2013-178980 filed 30 Aug.2013, and JP Patent Application No. 2014-165752 filed 18 Aug. 2014, theentire contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to a gas concentration measuringapparatus which works to measure the concentration of a gas componentsuch as NOx contained in exhaust gas from automotive vehicles.

BACKGROUND ART

A gas measuring apparatus which is equipped with a gas sensor exposed toexhaust gas from an automotive vehicle and a calculating portion whichuses an output of the gas sensor to calculate the concentration of NOxcontained in the exhaust gas (see Patent Literature 1, as listed below).

The gas sensor is equipped with a solid electrolyte body which hasoxygen ion conductivity and a plurality of electrodes formed on bothsurfaces of the solid electrolyte body. The solid electrolyte body andthe electrodes form three cells: a pump cell, a monitor cell, and asensor cell. The pump cell is a cell which reduces the concentration ofgas. The monitor cell is a cell which detects the concentration ofoxygen slightly remaining in the gas whose concentration of oxygen hasbeen reduced by the pump cell. The sensor cell is a cell which measuresthe sum of concentrations of oxygen and a given gas component (NOx)contained in the gas whose concentration of oxygen has been reduced bythe pump cell.

An amount of electric current (i.e., a monitor cell current Im) whichcorresponds to the concentration of oxygen remaining in the gas flowsthrough the monitor cell. An electric current (i.e., a sensor cellcurrent Is) also flows through the sensor cell. The sensor cell isactive both to oxygen and to the given gas component. The sensor cellcurrent Is, therefore, includes a component (i.e., an oxygen dependentcurrent Iso) arising from the concentration of oxygen and a component(i.e., a given gas dependent current Ix) arising from the concentrationof the given gas component. The accurate determination of theconcentration of the given gas component required elimination of theoxygen dependent current Iso from the sensor cell current Is toprecisely derive the given gas dependent current Ix.

It is, however, usually impossible to directly measure the oxygendependent current Iso. The above gas concentration measuring apparatus,therefore, uses the monitor cell current Im instead of the oxygendependent current Iso. Specifically, the monitor cell current Im, likethe oxygen dependent current Iso, has a correlation with theconcentration of oxygen. The monitor cell current Im is, thus,subtracted from the sensor cell current Is(=Iso+Ix) to obtain anapproximate value of the given gas dependent current Ix. The approximatevalue is used to calculate the concentration of the given gas component.This operation is made by the above calculating portion.

CITATION LIST Patent Literature

-   PATENT LITERATURE 1 Japanese Patent No. 3979240

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

It, however, may be impossible for the above gas measuring apparatus toaccurately measure the concentration of the given gas component.Specifically, the electrode constituting the monitor cell is made ofmetal, for example, Pt—Au alloy which reduces only oxygen molecules. Theelectrode making the sensor cell is made of metal, for example, Pt—Rhalloy which reduces both the oxygen molecules and the given gascomponent. The monitor cell and the sensor cell are different inmaterial of the electrodes from each other and thus different insensitivity to the concentration of oxygen from each other.Additionally, there is also a manufacturing variation in area of theelectrodes. This may result in a difference between the monitor cellcurrent Im and the oxygen dependent current Iso, which leads to adifficulty in subtracting the monitor cell current Im from the sensorcell current Is(=Ix+Iso) to accurately calculate the given gas dependentcurrent Ix for determining the concentration of the given gas component.

The invention was made against such a background and provides a gasconcentration measuring apparatus which is capable of determining theconcentration of the given gas component with enhanced accuracy.

Means for Solving the Problem

One aspect of the invention is a gas concentration measuring apparatuswhich comprises a gas sensor exposed to gas and a calculating portionwhich uses an output from the gas sensor to calculate a concentration ofa given gas component contained in the gas, characterized in that thegas sensor includes a gas chamber into which the gas is introduced, areference gas chamber into which a reference gas is introduced, a solidelectrolyte body which is disposed between the gas chamber and thereference gas chamber and has oxygen ion conductivity, and a pluralityof electrodes disposed on both surfaces of the solid electrolyte body,in that the solid electrolyte body and the electrodes constitute a pumpcell which works to regulate an oxygen concentration of the gas in thegas chamber, a monitor cell through which an amount of currentcorresponding to the oxygen concentration of the gas flows, and a sensorcell through which a current that is the sum of an amount of currentcorresponding to the oxygen concentration and an amount of currentcorresponding to a concentration of the given gas component in the gasflows, in that the calculating portion works to correct a value of amonitor cell current Im that is the current flowing through the monitorcell to bring it close to a value of an oxygen dependent current Isowhich is a component of a sensor cell current Is that is the currentflowing through the sensor cell and which arises from a concentration ofoxygen, and also subtract a corrected value Im′ thereof from the sensorcell current Is to determine a given gas dependent current Ix that is acomponent of the sensor cell current Is which arises from theconcentration of the given gas component to calculate the concentrationof the given gas component in the gas.

Effect of the Invention

The calculating portion of the gas concentration measuring apparatusworks to correct the monitor cell current Im to bring it close to theoxygen dependent current Iso when determining the concentration of thegiven gas component. This enhances the accuracy in calculating theconcentration of the given gas component. Specifically, the sensor cellcurrent Is, as described above, includes the given gas dependent currentIx that is a component arising from the concentration of the given gascomponent and the oxygen dependent current Iso. The above correctionoperation is, therefore, made to approximate the monitor cell currentIm′ after being corrected to the oxygen dependent current Iso.Subtraction of the monitor cell current Im′ from the sensor cell currentIs(=Ix+Iso), thus, derives a relation of Is−Im′=(Ix+Iso)−Im′≈Ix, whichenables the given gas dependent current Ix to be calculated accurately.This results in an enhanced accuracy in determining the concentration ofthe given gas component.

The present invention, as described above, provides the gasconcentration measuring apparatus which is capable of calculating theconcentration of the given gas component with enhanced accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a gas sensor in the first embodiment.

FIG. 2 is a II-II sectional view in FIG. 1.

FIG. 3 is a III-III sectional view in FIG. 1.

FIG. 4 is an exploded perspective view of a gas sensor in the firstembodiment.

FIG. 5 is a graph which represents a relation among a pump cell voltage,a monitor cell current, and a sensor cell current when gas containingNOx is measured in the first embodiment.

FIG. 6 is a graph which represents breakdowns of a sensor cell currentand a monitor cell current when Vp is set to about 0.37V in FIG. 5.

FIG. 7 is a graph which represents a relation among a pump cell voltage,a monitor cell current, and a sensor cell current when gas notcontaining NOx is measured in the first embodiment.

FIG. 8 is a graph which represents a correlation between a monitor cellcurrent and a sensor cell current when gas not containing NOx ismeasured in the first embodiment.

FIG. 9 is a graph which represents a relation among a pump cell voltageVp, Im, Is, Im−Is, and Im−aIs when gas not containing NOx is measured inthe first embodiment.

FIG. 10 is a graph which represents breakdowns of a sensor cell currentand a monitor cell current when gas containing NOx is measured at Vp setto about 0.2V in the first embodiment.

FIG. 11 is a correlation between the monitor cell current and the sensorcell current in FIG. 10.

FIG. 12 is a graph which represents a relation among a pump cellvoltage, a monitor cell current, and a sensor cell current in terms ofV1, V2, Im₁, Im₂, Is₁, and Is₂ when gas containing NOx is measured.

FIG. 13 is a graph which represents a breakdown of a sensor cell currentwhen a pump cell voltage Vp is set to V1 or V2 in FIG. 12.

FIG. 14 is a graph which represents a breakdown of a monitor cellcurrent when a pump cell voltage Vp is set to V1 or V2 in FIG. 12.

FIG. 15 is a graph which represents a relation among a pump cellvoltage, a monitor cell current, and a sensor cell current in terms ofvalues of V0 and V3 when gas containing NOx is measured in the thirdembodiment.

FIG. 16 is a graph which represents breakdowns of | a sensor cellcurrent | and | a monitor cell current | when a pump cell voltage Vp isset to V3, as illustrated in FIG. 15.

FIG. 17 is a graph which represents a relation among a pump cellvoltage, a monitor cell current, and a sensor cell current in terms ofvalues of V1 and V2 when gas not containing NOx is measured in thefourth embodiment.

FIG. 18 is a graph which shows breakdowns of Im₁ and Is₁ in the fourthembodiment.

FIG. 19 is a graph which shows breakdowns of Im₂ and Is₂ in the fourthembodiment.

FIG. 20 is a graph which shows breakdowns of Im and Is when gascontaining NOx is measured in the fourth embodiment.

FIG. 21 is a graph which represents breakdowns of sensor cell currentsIm₃ and Is₄ when a pump cell voltage is set to V3 or V4 in the fifthembodiment.

FIG. 22 is a graph which represents breakdowns of monitor cell currentsIm₃ and Im₄ when a pump cell voltage is set to V3 or V4 in the fifthembodiment.

EMBODIMENT FOR CARRYING OUT THE INVENTION

The above described gas concentration measuring apparatus may bedesigned as, for example, a NOx concentration measuring apparatus whichmeasures the concentration of NOx contained in exhaust emissions fromautomotive vehicles.

Embodiment

Embodiment 1

An embodiment of the above gas concentration measuring apparatus will beexplained using FIGS. 1 to 11. The gas concentration measuring apparatus1 of this embodiment is, as illustrated in FIGS. 1 to 4, equipped with agas sensor 10 and a calculating portion 11. The gas sensor 10 has thetop end 100 exposed to the gas g. The calculating portion 11 uses anoutput current from the gas sensor 10 to calculate the concentration ofa gas component contained in the gas g.

The gas sensor 10 includes a gas chamber 7, a reference gas chamber 12,a solid electrolyte body 2, and a plurality of electrodes 8. The gas gis admitted into the gas chamber 7. A reference gas such as atmosphericair is admitted into the reference gas chamber 7. The solid electrolytebody 2 is interposed between the gas chamber 7 and the reference gaschamber 12. The solid electrolyte body 2 is made from material havingoxygen ion conductivity. The electrodes 8 are formed on both surfaces ofthe solid electrolyte body 2.

The solid electrolyte body 2 and the electrodes 8, as illustrated inFIGS. 1 to 3, form a pump cell 3, a monitor cell 4, and a sensor cell 5.The pump cell 3 is a cell working to regulate the concentration ofoxygen in the gas g within the gas chamber 7. The monitor cell 4 is acell through which an electric current flows as a function of theconcentration of oxygen in the gas g. The sensor cell 5 is a cellthrough which the sum of an amount of current corresponding to theconcentration of oxygen in the gas g and an amount of currentcorresponding to the concentration of a given gas component in the gas gflows.

The monitor cell current Im flows through the monitor cell 4. The sensorcell current Is flows through the sensor cell 5. The sensor cell currentIs includes an oxygen dependent current Iso that is a componentdeveloped by the concentration of oxygen and a given gas dependentcurrent Ix that is a component developed by the concentration of thegiven gas component (see FIG. 6). The calculating portion 11 works toperform a correction operation to bring the value of the monitor cellcurrent Im close to the value of the oxygen dependent current Iso. Thevalue of the corrected monitor cell current Im′ is subtracted from thesensor cell current Is to derive the given gas dependent current Ix. Thevalue of the given gas dependent current Ix is used to calculate theconcentration of the given gas component contained in the gas g.

The gas concentration measuring apparatus 1 of this embodiment is a NOxconcentration measuring apparatus designed to measure the concentrationof NOx contained in exhaust gas from an automotive vehicle.Specifically, the gas g is the exhaust gas from the automotive vehicle.The given gas component is NOx. The whole of the gas sensor 10 isinstalled in a cylindrical case, not shown, and installed in an exhaustpipe of the automotive vehicle. The top end 100 of the gas sensor 10 isinserted into the exhaust pipe, while the rear end thereof is exposed toatmospheric air.

The electrode 8, as illustrated in FIGS. 1 to 4, includes a pumpelectrode 8 p, a monitor electrode 8 m, and a sensor electrode 8 s whichare formed on a major surface 21 of the solid electrolyte body 2 facingthe gas chamber 7 and a reference electrode 8 b which is formed on amajor surface of the solid electrolyte body 2 facing the reference gaschamber 12. The solid electrolyte body 2, the pump electrode 8 p, andthe reference electrode 8 b form the pump cell 3. The solid electrolytebody 2, the monitor electrode 8 m, and the reference electrode 8 b formthe monitor cell 4. The solid electrolyte body 2, the sensor electrode 8s, and reference electrode 8 b form the sensor cell 5.

How to measure the concentration of NOx in the gas g will be describedbelow. The gas g, as illustrated in FIG. 1, passes through the diffusionresistance layer 13 and then enters the gas chamber 7. The gas gcontains oxygen molecules which are discharged using the pump cell 3.Specifically, a dc voltage is applied between the reference electrode 8b and the pump electrode 8 p so as to place the reference electrode 8 bat a higher potential. This causes the oxygen molecules to be reduced onthe pump electrode 8 p into oxygen ions which are then discharged intothe reference gas chamber 12 through pumping action. The concentrationof oxygen in the gas chamber 7 is controlled by regulating the degree ofdc voltage applied to the pump cell 3.

The gas g whose concentration of oxygen has been decreased is deliveredto the monitor cell 4 and the sensor cell 5. The gas g still containsthe oxygen molecules which have not been removed by the pump cell 3. Theconcentration of such oxygen molecules is measured by the monitor cell4. The dc voltage is, as illustrated in FIG. 3, applied between thereference electrode 8 b and the monitor electrode 8 m of the monitorcell 4 so as to place the reference electrode 8 b at a higher potential.This will cause the oxygen molecules contained in the gas g to bereduced into oxygen ions which are then discharged into the referencegas chamber 12 through pumping action. The monitor electrode 8 m is, asdescribed later, made of a Pt—Au cermet electrode which is inactive todecomposition of NOx. The current (i.e., the monitor cell current Im)flowing through the monitor cell 4, thus, depends only on theconcentration of oxygen molecules contained in the gas g, not theconcentration of NOx.

The dc voltage is also applied between the reference electrode 8 b andthe sensor electrode 8 s of the sensor cell 5 so as to place thereference electrode 8 b at a higher potential. The sensor electrode 8 sis, as described later, made of a Pt—Rh cermet electrode which isinactive to decomposition of NOx. The oxygen molecules and NOx moleculesare, thus, reduced on the sensor electrode 8 s into oxygen ions whichare then discharged into the reference gas chamber 12 through thepumping action. The current (i.e., the sensor cell current Is) flowingthrough the sensor cell 5 is, therefore, depends on both theconcentration of the oxygen molecules and the concentration of NOxmolecules.

When the voltage (i.e., the pump cell voltage Vp) applied to the pumpcell 3 is, as illustrated in FIG. 5, decreased, it will cause themonitor cell current Im and the sensor cell current Is to increase. Thisis because a drop in the pump cell voltage Vp usually results in adecrease in amount of oxygen discharged from the gas g by the pump cell3, so that the centration of oxygen in the gas g rises, thus causing theamount of oxygen pumped out by the monitor cell 4 and the sensor cell 5to increase the amount of oxygen ion flowing through the monitor cell 4and the sensor cell 5, which will lead to increases in the monitor cellcurrent Im and the sensor cell current Is.

For measurement of the concentration of NOx, the pump cell voltage Vp isset to a given level, e.g., about 0.37V.

The sensor cell 5 is, as described above, active to both the given gascomponent and oxygen, so that the sensor cell current Is, as can be seenfrom FIG. 6, includes the given gas dependent current Ix arising fromthe concentration of the given gas component and the oxygen dependentcurrent Iso arising from the concentration of oxygen. The monitor cell 4is active only to oxygen, so that the monitor cell current Im does nothave a component developed by the concentration of the given gascomponent.

The monitor cell current Im is, as can been seen in FIG. 6, slightlylower than the oxygen dependent current Iso. In other words, the monitorcell 4 and the sensor cell 5 are different in sensitivity to theconcentration of oxygen from each other. This is because the electrodes8 of the monitor cell 4 and the sensor cell 5 are different in materialfrom each other, and there is a production variability in size of areasbetween them. This will lead to an error in subtracting the monitor cellcurrent Im from the sensor cell current Is to derive the given gasdependent current Ix in calculating the concentration of the given gascomponent.

Accordingly, in this embodiment, the correction operation is performedto bring the monitor cell current Im close to the oxygen dependentcurrent Iso. Specifically, the calculating portion 11 (see FIG. 1)stores therein a value derived by dividing the oxygen dependent currentIso by the monitor cell current Im as a sensitivity ratio a andsubstituting the sensor cell current Is, the monitor cell current Imwhich have been measured, and the sensitivity ratio a into an equationbelow to derive the given gas dependent current Ix.Is−aIm=Ix

The given gas dependent current Ix, as calculated above, is used todetermine the concentration of the given gas component. Note that sinceIs−aIm=(Ix+Iso)−Iso/Im×Im=Ix, Is−aIm is equal to Ix.

Because of a unit-to-unit variability of the gas sensor 10, thesensitivity ratio a needs to be measured and stored. For instance, whenthe gas concentration measuring apparatus 1 is produced and shipped, theatmospheric air which does not contain the given gas component ismeasured as the gas g. Using resulting values of the sensor cell currentIs and the monitor cell current Im, the sensitivity ratio a iscalculated.

When the gas g contains the given gas component, it is impossible todirectly measure the oxygen dependent current Iso, however, when the gasg does not contain the given gas component, a relation of the sensorcell current Is=the oxygen dependent current Iso will be met, thusmaking it possible to directly measure the oxygen dependent current Iso.The sensitivity ratio a is, therefore, derived using the followingequation.a=Iso/Im=Is/Im

The graph of FIG. 7 shows that when the atmospheric air which does notcontain the given gas component is measured, it will cause the sensorcell current Is to be slightly higher than the monitor cell current Im.This is because the sensor cell 5 is higher in sensitivity to oxygenthan the monitor cell 4. The sensitivity ratio a(=Is/Im) is often higherthan one.

In this embodiment, in a condition where the gas g does not contain thegiven gas component, the pump cell voltage Vp is changed stepwise tomake conditions within the gas chamber 7 which are different in value ofthe concentration of oxygen. In each of the conditions, the monitor cellcurrent Im and the sensor cell current Is are measured. FIG. 8represents a correlation between the monitor cell current Im and thesensor cell current Is, as derived in the above way.

When the pump cell voltage Vp is lower, the concentration of oxygen inthe gas g (i.e., the atmospheric air) increases, so that the monitorcell current Im and the sensor cell current Is increase. Conversely,when the pump cell voltage Vp is higher, the concentration of oxygen inthe gas g decreases, so that the monitor cell current Im and the sensorcell current Is decreases. FIG. 8 shows that values of the monitor cellcurrent Im and the sensor cell current Is, as measured when theconcentration of oxygen in the gas g is changed, appear on a straightline. In this embodiment, the slope of such a straight line is used asthe sensitivity ratio a(=Is/Im). The determination of the slope of thestraight line is achieved using the least-square technique.

The sensitivity ratio a, as described above, depends upon the individualvariability of the gas sensor 10. The sensitivity ratio a, that is, theslope of the straight line in FIG. 8 has a value of 1 to 2 in most gassensors 10.

The graph of FIG. 9 represents a relation among the pump cell voltageVp, Im, Is, Is−Im, and Is−aIm when the gas g which does not contain NOxis measured. The graph shows that a value derived by subtracting themonitor cell current Im from the sensor cell current Is (i.e., Is−Im)will be a great value equivalent to about ±6 ppm, as converted in termsof concentration of NOx, in a range of 0.35 to 0.39V of the pump cellvoltage Vp (i.e., a range where the pump cell voltage Vp will vary whenthe concentration of NOx is measured). In contrast, a value derived bymultiplying the monitor cell current Im by the sensitivity ratio a inthe correction operation and subtracting it from the sensor cell currentIs (i.e., Is−aIm) will be a small value equivalent to ±1 ppm or less, asconverted in terms of concentration of NOx which is lower than Is−Im ina range of 0.35 to 0.39V of the pump cell voltage Vp.

When the gas g containing NOx is measured, the sensor cell current Is,as illustrated in FIG. 6, will be the sum of the given gas dependentcurrent Ix and the oxygen dependent current Iso. In this embodiment, theIso and aIm are adjusted to be substantially equal to each other, sothat the concentration of NOx, as derived from Iso−aIm, will be ±1 ppmor less, thus resulting in enhanced accuracy in calculating the givengas dependent current Ix.

The characteristics of the electrode 8 of the gas sensor 10 changes withtime, so that the sensitivity ratio a will change with time. It is,thus, necessary to measure the sensitivity ratio a at regular intervalsto update and store the value of the sensitivity ratio a.

Before the gas concentration measuring apparatus 1 is installed in theautomotive vehicle, it is, as described above, possible to measure thegas g which does not contain the given gas component, so that a relationof Is=Iso is met. A relation of a=Iso/Im=Is/Im is, thus, derived, whichenables the sensitivity ratio a to be calculated accurately.

However, after the gas concentration measuring apparatus 1 is mounted inthe automotive vehicle, it is impossible to measure the atmospheric airwhich does not contain the given gas component (i.e., NOx). When the gasg contains the given gas component, it is impossible to directly measurethe oxygen dependent current Iso, thus resulting in an error inmeasuring the sensitivity ratio a(=Iso/Im).

Consequently, in this embodiment, an approximate value a′ of thesensitivity ratio a is calculated at regular intervals. Specifically,when the concentration of NOx is measured, the pump cell voltage Vp is,as described above, set to approximately 0.37V, while when theapproximate value a′ of the sensitivity ratio a is calculated, the pumpcell voltage Vp is set to a value lower than 0.37V, for example,approximately 0.2V. This results in a decrease in amount of oxygenmolecules discharged by the pump cell 3, so that the concentration ofoxygen in the gas g will increase which is greatly higher than theconcentration of the given gas component, so that a percentage of theoxygen dependent current Iso in the sensor cell current Is, as indicatedin FIG. 10, will be much higher than that of the given gas dependentcurrent Ix. It is, thus, possible to ignore the given gas dependentcurrent Ix, thereby enabling, as illustrated in FIG. 11, the approximatevalue a′(=Is/Im) to be derived as being substantially equal to thesensitivity ratio a(=Iso/Im).

After the approximate value a′ is calculated, an equation below is usedto determine the concentration of the given gas component. Specifically,measured values of the sensor cell current Is, the monitor cell currentIm, and the approximate value a′ are substituted into the equation belowto determine an approximate value Ix′ of the given gas dependent currentIx. Use of the approximate value Ix′ enables the concentration of thegiven gas component to be calculated with high accuracy.Is−a′Im=Ix′

The structure of the gas sensor 10 will be described below in detail.The gas sensor 10 is, as illustrated in FIGS. 1 to 4, equipped with thesingle solid electrolyte body 2. The pump electrode 8 p, the monitorelectrode 8 m, the sensor electrode 8 s, and the reference electrode 8 bare formed in or on the solid electrolyte body 2. Specifically, in thisembodiment, the single solid electrolyte body 2 is used to make threecells: the pump cell 3, the monitor cell 4, and the sensor cell 5.

The gas sensor 10, as illustrated in FIGS. 1 and 4, includes aninsulating plate 191 made of ceramic, a first spacer 192 which issheet-like and defines the gas chamber 7, the solid electrolyte body 2,a second spacer 193 which is sheet-like and defines the reference gaschamber 12, and a heater 6 which are all stacked in a width-wisedirection of the heater 6 (i.e., z-direction).

The first spacer 192, as illustrated in FIG. 4, has formed therein afirst cut-out 79 which defines the gas chamber 7. The first spacer 192also has the diffusion resistance layer 13 through which the gas g isadmitted from the exhaust pipe to the gas chamber 7. The diffusionresistance layer 13 works to limit the rate at which the gas g flows.

The second spacer 193 has a second cut-out 129 formed therein. Thesecond cut-out 129 defines the reference gas chamber 12. The secondcut-out 129 communicates with an external space in which atmospheric airexists. The first spacer 192 and the second spacer 193 are made ofinsulating material such as alumina.

The pump electrode 8 p and the monitor electrode 8 m are made ofmetallic material which is low in activity to decompose NOx.Specifically, the electrodes 8 p and 8 m are made of a porous cermetelectrode which mainly contains gold Au and platinum Pt. A content of Auin the electrodes 8 p and 8 m is 1 to 10 by weight percent. The sensorelectrode 8 s is made of metallic material which is high in activity todecompose NOx. Specifically, the sensor electrode 8 s is made of aporous cermet electrode which mainly contains Platinum Pt and rhodiumRh.

The electrodes 8 have formed thereon leads 16 which define currentpaths. The solid electrolyte body 2, the first spacer 192, and theinsulating plate 191 have formed therein through-holes 17 passingtherethrough in the z-direction. Metallic plugs are formed in thethrough holes 17. The insulating plate 191 has formed on a surfacethereof a plurality of pads 15 for electrical connections with anexternal device. Each of the pads 15 electrically connects with one ofthe electrodes 8 m, 8 s, and 8 p through the plug 16.

The heater 6 is made up of a ceramic heater sheet 62, a heater electrode63 which is formed on the surface of the heater sheet 62 and works toproduce heat when electrically energized, and an insulating layer 61which covers the heater electrode 63. The heater 6 is designed to havethe heater electrode 63 which produces heat when supplied withelectricity from an external device to heat the above described cells 3,4, and 5 up to an activation temperature thereof. The second spacer 193,the insulating layer 61, and the heater sheet 62 have through-holes 170formed therein. The heater sheet 62 has pads 18 formed on the surfacethereof. Metallic plugs are formed in the through-holes 170 to establishelectrical connections of the heater electrode 63 and the referenceelectrode 8 b with the pads 18.

The center where the heater electrode 63 produces heat is located closerto the pump cell 3. In other words, the cells 3, 4, and 5 are heatedusing the heater electrode 63 so that the temperature of the pump cell 3becomes higher than those of the monitor cell 4 and the sensor cell 5.

The gas chamber 7 of this embodiment is, as illustrated in FIGS. 1 and2, integrally formed to have a space which is entirely constant both inthe z-direction and in a width-wise direction (i.e., the y-direction)perpendicular to both the x- and z-directions from where the pump cell 3is formed to where the monitor cell 4 and the sensor cell 5 are formed.In other words, there is not an object, such as an orifice or apartition, which decreases the space in the z-direction or they-direction from where the pump cell 3 is formed to where the monitorcell 4 and the sensor cell 5 are formed in the gas chamber 7, therebycausing the gas g to flow without being subjected to diffusionlimitation from where the pump cell 3 is formed to where the monitorcell 4 and the sensor cell 5 are formed.

The distance L1, as illustrated in FIG. 2, between the pump electrode 8p and the monitor electrode 8 m and the distance L2 between the pumpelectrode 8 p and the sensor electrode 8 s are equal to each other inthe x-direction.

The reference electrode 8 b is, as can be seen in FIGS. 1 and 4, acommon electrode. In other words, portions of the reference electrode 8b which constitute the pump cell 3, the monitor cell 4, and the sensorcell 5 are integrally formed.

The operations and beneficial effects of this embodiment will bedescribed. The calculating portion 11 of the gas concentration measuringapparatus 1 works to correct the monitor cell current Im to bring itclose to the oxygen dependent current Iso when determining theconcentration of the given gas component. This enhances the accuracy incalculating the concentration of the given gas component. Specifically,the sensor cell current Is, as described above, includes the given gasdependent current Ix that is a component arising from the concentrationof the given gas component and the oxygen dependent current Iso. Theabove correction operation is, therefore, made to approximate themonitor cell current Im′ after corrected to the oxygen dependent currentIso. Subtraction of the monitor cell current Im′ from the sensor cellcurrent Is(=Ix+Iso), thus, derives a relation of Is−Im′=(Ix+Iso)−Im′≈Ix,which enables the given gas dependent current Ix to be calculatedaccurately. This results in an enhanced accuracy in determining theconcentration of the given gas component.

The calculating portion 11 also uses the following equation to calculatethe given gas dependent current Ix in the sensor cell current Is.Is−aIm=Ix

The given gas dependent current Ix, as derived above, is used todetermine the concentration of the given gas component. This facilitatesand ensures the calculation of the given gas dependent current Ix in thesensor cell current Is.

In this embodiment, the sensor cell current Is and the monitor cellcurrent Im are, as illustrated in FIGS. 7 and 8, measured in thecondition where the gas g does not contain the given gas component. Insuch a condition, since the sensor cell current Is does not contain thegiven gas dependent current Ix, a relation of the sensor cell currentIs=the oxygen dependent current Iso is met. It is, therefore, possibleto use the following equation to calculate the sensitivity ratio aeasily and accurately.a=Iso/Im=Is/Im

In this embodiment, the calculation of the sensitivity ratio a isachieved, as illustrated in FIGS. 7 and 8, by changing the pump cellvoltage Vp. This makes a plurality of conditions which have differentvalues of the concentration of oxygen in the gas g within the gaschamber 7. In each condition, the sensor cell current Is and the monitorcell current Im are measured to derive the sensitivity ratio a.

The use of the plurality of measured data enables the sensitivity ratioa to be determined more accurately.

In this embodiment, after the gas sensor 10 is installed in theautomotive vehicle, the gas g containing the given gas component is usedto calculate the approximate value a′ of the sensitivity ratio a. Thiscalculation is achieved by setting the pump cell voltage Vp to be lowerthan that when the concentration of the given gas component isdetermined. The approximate value a′ is derived using the followingequation.Is/Im=a′

It makes it possible to use the gas g containing the given gas componentto calculate the approximate value a′ that approximates the sensitivityratio a. The sensitivity ratio a, as already described, changes withtime, thus requiring the need for updating the sensitivity ratio acyclically. Once the gas concentration measuring apparatus is mounted inthe automotive vehicle, it is impossible to measure the atmospheric airas the gas g which does not contain the given gas component (NOx). Inthis embodiment, it is possible to derive the value approximating thesensitivity ratio a even when the gas g contains the given gascomponent, thus ensuring the accuracy in measuring the concentration ofthe given gas component even if the sensitivity ratio a changes withtime.

It is preferable that the calculation of the approximate value a′ ismade when the concentration of the given gas component is as low aspossible, like when the engine of the vehicle is undergoing a fuel cut.

The gas sensor 10 of this embodiment, as illustrated in FIGS. 1 and 2,is formed integrally so as to have a width which is entirely constant inthe z-direction from where the pump cell 2 is formed to where themonitor cell 4 and the sensor cell 5 are formed. In other words, the isnot an object, such as an orifice or a partition, which decreases thespace in the z-direction or the y-direction from where the pump cell 3is formed to where the monitor cell 4 and the sensor cell 5 are formedwithin the gas chamber 7, thereby causing the gas g to flow withoutbeing subjected to diffusion limitation from where the pump cell 3 isformed to where the monitor cell 4 and the sensor cell 5 are formed.This achieves quick detection of a change in concentration of the givengas component, thus enhancing the response rate of the gas sensor 10.

In this embodiment, the single solid electrolyte body 2 is, asillustrated in FIGS. 1 to 4, shared among the pump cell 3, the monitorcell 4, and the sensor cell 5. This eliminates the need for using aplurality of solid electrolyte bodies, thus resulting in a decrease inproduction cost of the gas sensor 10.

It is, thus, possible to provide a gas concentration measuring apparatuswhich is capable of measuring the concentration of the given gascomponent more accurately.

Second Embodiment

In the following embodiments, the same reference symbols used in thedrawings as those in the first embodiment will refer to the same partsunless otherwise specified.

This embodiment is an example of a modification of how to calculate theapproximate value a′ of the sensitivity ratio a using the gas gcontaining the given gas component. A first voltage V1 and a secondvoltage V2 which are different in level from each other are, asillustrated in FIG. 12, applied to the pump cell 3. The monitor cellcurrent Im₁ and the sensor cell current Is₁ when the first voltage V1 isapplied are measured. The monitor cell current Im₂ and the sensor cellcurrent Is₂ when the second voltage V2 is applied are measured. Themeasured values Im₁, Is₁, Im₂, and Is₂ are used to determine theapproximate value a′ according to an equation below.(Is ₁ −Is ₂)/(Im ₁ −Im ₂)=a′  (1)

The sensor cell current Is₁ and Is₂, as can be seen from FIG. 13, eachinclude the given gas dependent current Ix that is a component arisingfrom the concentration of the given gas component and the oxygendependent current Iso that is a component arising from the concentrationof oxygen. The sensor cell current Is₁ and Is₂ are values measured bychanging the pump cell voltage Vp, so that values of the oxygendependent current Iso are different from each other. Specifically, whenthe pump cell voltage Vp is low (i.e., the first voltage V1), theconcentration of oxygen in the gas g will be high, so that the value ofthe oxygen dependent current Iso will be high. Alternatively, when thepump cell voltage Vp is high (i.e., the second voltage V2), theconcentration of oxygen in the gas g will be low, so that the value ofthe oxygen dependent current Iso will be low.

In contrast, the concentration of the given gas component is notaffected by the pump cell voltage Vp, so that the given gas dependentcurrent Ix does not change between when Vp=V1 and when Vp=V2. Theelimination of the given gas dependent current Ix is, therefore,achieved by deriving Is₁−Is₂ in the above equation (1). The oxygendependent current Iso(Iso₂) when Vp=V2 will be much lower than theoxygen dependent current Iso(Iso₁) when Vp=V1.

Similarly, as illustrated in FIG. 14, when the pump cell voltage Vp islow (i.e., the first voltage V1), the monitor cell current Im₁ will behigh. Alternatively, when the pump cell voltage Vp is high (i.e., thesecond voltage V2), the monitor cell current Im₂ will be low.

As described above, use of the above equation (1) eliminates the givengas dependent current Ix, so that the approximate value a′ is notaffected by the concentration of the given gas component, thus enablingthe approximate value a′ which is very close to the sensitivity ratio ato be derived.

After the approximate value a′ is calculated, the concentration of thegiven gas component is, like in the first embodiment, determinedaccurately using an equation below.Is−a′Im=Ix′

This embodiment offers the same arrangements and beneficial effects asthose in the first embodiment.

Third Embodiment

This embodiment is an example of a modification of how to calculate theapproximate value a′ of the sensitivity ratio a using the gas gcontaining the given gas component. In this embodiment, the pump cellvoltage Vp for use in calculating the approximate value a′ is, asillustrated in FIG. 15, set to a level V3 which is higher than a levelV0 used to calculate the concentration of the given gas component. Thiscauses the NOx that is the given gas component to be decomposed on thepump electrode 8 p (see FIGS. 1 and 2). The monitor cell current Im andthe sensor cell current Is are then measured. Since the given gascomponent in the gas g will be decomposed, the sensor cell current Is,as illustrated in FIG. 16, does not include the given gas dependentcurrent Ix, but contains only the oxygen dependent current Iso. Thisenables the approximate value a′ of the sensitivity ratio a to becalculated according to an equation below.a′=Iso/Im=Is/Im

Too high a level of the pump cell voltage Vp will cause H₂O contained inthe gas g to be decomposed on the pump electrode 8 p, so that H₂ gas isgenerated. The H₂ gas then flows on the monitor cell 4 and the sensorcell 5, thereby causing the monitor cell current Im and the sensor cellcurrent Is to have minus values. When such values are used to calculatethe sensitivity ratio, it will cause the sensitivity to H₂ to bederived. Consequently, the determination of the sensitivity ratio a toO₂ requires use of plus values of the monitor cell current Im and thesensor cell current Is.

This embodiment offers the same arrangements and beneficial effects asthose in the first embodiment.

Fourth Embodiment

This embodiment is an example of a modification of how to calculate thegiven gas dependent current Ix. It is known that an offset currentI_(off) usually flows through the monitor cell 4 and the sensor cell 5regardless of presence of oxygen and the given gas component. The offsetcurrent I_(off) is thought of as arising from electron conductionoccurring between the solid electrolyte body 2 and the electrode 8. Inthis embodiment, the monitor cell current Im is corrected under theassumption that the monitor cell current Im and the sensor cell currentIs contain the offset current I_(off). This will be described below indetail.

In this embodiment, when the gas concentration measuring apparatus 1 isshipped, the atmospheric air not containing NOx is used to measure thesensor cell current Is and the monitor cell current Im. Specifically, afirst voltage V1 and a second voltage V2 which are different in levelfrom each other are applied to the pump cell 3 (see FIG. 17). The sensorcell current Is₁ and the monitor cell current Im₁ when the first voltageV1 is applied are measured. The sensor cell current Is₂ and the monitorcell current Im₂ when the second voltage V2 is applied are measured.

When the first voltage V1 which is lower than the second voltage V2 isapplied to the pump cell 3, the concentration of O₂ within the gaschamber 7 will be higher than that when the second voltage V2 which ishigher than the first voltage V1 is applied to the pump cell 3.Therefore, values of the sensor cell current Is₁ and the monitor cellcurrent Im₁ when the first voltage V1 is applied will be greater thanthose of the sensor cell current Is₂ and the monitor cell current Im₂when the second voltage V2 is applied.

The monitor cell currents Im₁ and Im₂ (see FIG. 19) and the sensor cellcurrent Is₁ and Is₂, as illustrated in FIGS. 18 and 19, each contain theoffset current I_(off). The offset current I_(off) does not depend uponthe concentration of oxygen, so that values of the offset currentsI_(off) contained in the currents Im₁, Im₂, Is₁, and Is₂ aresubstantially identical with each other.

Components of the monitor cell current Im₁ and the sensor cell currentIs₁ which are sensitive to the concentration of oxygen are onlycomponents (Im₁−I_(off)) and (Is₁−I_(off)) thereof from which the offsetcurrent I_(off) is removed. A ratio of Im₁−I_(off) to Is₁−I_(off) isdefined as a subtracted value sensitivity ratio β.β=(Is ₁ −I _(off))/(Im ₁ −I _(off))

The subtracted value sensitivity ratio β is constant even when the pumpcell voltage Vp is changed to change the concentration of oxygen in thegas chamber 7 (see FIG. 19). The subtracted value sensitivity ratio βwhen the pump cell voltage Vp is set to the above second voltage V2 isgiven byβ=(Is ₂ −I _(off))/(Im ₂ −I _(off))

Therefore, the subtracted value sensitivity ratio β is generally definedby the following formula.β=(Is−I _(off))/(Im−I _(off))

It is apparent from FIGS. 18 and 19 that the sensor cell current Is₁ andI₂ are expressed byIs ₁=β(Im ₁ −I _(off))+I _(off)Is ₂=β(Im ₂ −I _(off))+I _(off)

Solving the above system of equations, β and I_(off) are expressed byβ=(Is ₁ −Is ₂)/(Im ₁ −Im ₂)I _(off)=(Is ₁ −βIm ₁)/(1−β)

The calculating portion 11 in this embodiment calculates the subtractedvalue sensitivity ratio β and the offset current I_(off) using the aboveequations and stores them.

When determining the concentration of NOx, the calculating portion 11,as illustrated in FIG. 20, measures the monitor cell current Im and thesensor cell current Is. The calculating portion 11 then corrects themeasured value of the monitor cell current Im to bring it close to thevalue of the oxygen dependent current Iso using the following equation.

Im′=β(Im−I _(off))+I _(off)

Subsequently, the calculating portion 11 subtracts the corrected valueIm′ of the monitor cell current Im from the sensor cell current Is usingan equation below to derive the given gas dependent current Ix.Ix=Is−Im′=Is−{β(Im−I _(off))+I _(off)}

By the above operations, the given gas dependent current Ix is derivedaccurately to determine the concentration of the given gas component.

This embodiment offers the same arrangements and beneficial effects asthose in the first embodiment.

Fifth Embodiment

This embodiment is an example where the gas g containing NOx is used todetermine the approximate value β′ of the subtracted value sensitivityratio β and the approximate value I_(off)′ of the offset currentI_(off). The subtracted value sensitivity ratio β and the offset currentI_(off) usually change with time. After the gas concentration measuringapparatus 1 is installed in the automotive vehicle, there are fewchances to measure the gas g which does not contain NOx. In thisembodiment, the gas g containing NOx is, therefore, used to measure theapproximate value β′ of the subtracted value sensitivity ratio β and theapproximate value I_(off)′ of the offset current I_(off) at regularintervals and update them.

When measuring the approximate values β′ and I_(off)′, a third voltageV3 and a fourth voltage V4 which are different in level from each otherare applied to the pump cell 3. The sensor cell current Is₃ and themonitor cell current Im₃ when the third voltage V3 is applied (see FIGS.21 and 22), and the sensor cell current Is₄ and the monitor cell currentIm₄ when the fourth voltage V4 is applied are measured. The approximatevalues β′ and I_(off)′ are calculated according to the followingequation.β′=(Is ₃ −Is ₄)/(Im ₃ −Im ₄)I _(off)′=(Is ₃ −β′Im ₃)/(1−β′)  (2)

The values of the sensor cell current Is₃ and Is₄ are derived when thegas g has the same the concentration of the given gas component, so thatvalues of the given gas dependent current Ix contained in the sensorcell current Is₃ and Is₄ are identical with each other. The values ofthe offset current I_(off) are also identical with each other. In thenumerator of the above equation (2), an operation of Is₃−Is₄ is made, sothat Ix and I_(off) will be eliminated. Similarly, as illustrated inFIG. 22, value of the offset current I_(off) contained in the monitorcell currents Im₃ and Im₄ are the same. In the denominator of the aboveequation (2), an operation of Im₃−Im₄ is made, so that I_(off) will beeliminated.

The use of the above equation (2) results in elimination of the offsetcurrent I_(off) and the given gas dependent current Ix contained in eachof the currents Is₃, Is₄, Im₃, and Im₄, thereby ensuring an enhancedaccuracy in calculating the approximate value β′ of the subtracted valuesensitivity ratio β.

When measuring the concentration of the given gas component (NOx), thecalculating portion 11 works to measure the sensor cell current Is andthe monitor cell current Im and then correct the value of the monitorcell current Im according to an equation below to bring it close to thevalue of the oxygen dependent current Iso.Im′=β′(Im−I _(off)′)+I _(off)′

Subsequently, the calculating portion 11 uses an equation below tosubtract the corrected value Im′ of the monitor cell current Im from thesensor cell current Is to derive the approximate value Ix′ of the givengas dependent current Ix and then uses the approximate value Ix′ todetermine the concentration of the given gas component.Ix′=Is−Im′=Is−{β′(Im−I _(off′))+I _(off′)}

In the above way, the approximate values β′ and I_(off′) which have beenupdated at regular intervals are used to calculate the approximate valueIx′ of the given gas dependent current Ix, thereby resulting in anenhanced accuracy in determining the concentration of gas regardless ofaging of the gas sensor 10.

This embodiment offers the same arrangements and beneficial effects asthose in the first embodiment.

EXPLANATION OF REFERENCE SYMBOLS

-   1 gas concentration measuring apparatus-   10 gas sensor-   11 calculating portion-   12 reference gas chamber-   2 solid electrolyte body-   3 pump cell-   4 monitor cell-   5 sensor cell-   7 gas chamber-   8 electrode-   Im monitor cell current-   Is sensor cell current-   Iso oxygen dependent current-   Ix given gas dependent current-   g gas

The invention claimed is:
 1. A gas concentration measuring apparatuswhich comprises a gas sensor exposed to gas and a calculating portionwhich uses an output from the gas sensor to calculate a concentration ofa given gas component contained in the gas, characterized in that thegas sensor includes a gas chamber into which the gas is introduced, areference gas chamber into which a reference gas is introduced, a solidelectrolyte body which is disposed between the gas chamber and thereference gas chamber and has oxygen ion conductivity, and a pluralityof electrodes disposed on both surfaces of the solid electrolyte body,in that the solid electrolyte body and the electrodes constitute a pumpcell which works to regulate an oxygen concentration of the gas in thegas chamber, a monitor cell through which an amount of currentcorresponding to the oxygen concentration of the gas flows, and a sensorcell through which a current that is the sum of an amount of currentcorresponding to the oxygen concentration and an amount of currentcorresponding to a concentration of the given gas component in the gasflows, in that the calculating portion works to correct a value of amonitor cell current Im that is the current flowing through the monitorcell to bring it close to a value of an oxygen dependent current Isowhich arises from an oxygen concentration and is a component of a sensorcell current Is that is the current flowing through the sensor cell andalso subtract a corrected value Im′ thereof from the sensor cell currentIs to determine a given gas dependent current Ix that is a component ofthe sensor cell current Is which arises from the concentration of thegiven gas component to calculate the concentration of the given gascomponent in the gas, in that the calculating portion applies a firstvoltage (V1) and a second voltage (V2) which have values different fromeach other to the pump cell in a condition where the gas does notcontain the given gas component and measures the monitor cell currentIm₁ and the sensor cell current Is₁ when the first voltage (V1) isapplied and the monitor cell current Im₂ and the sensor cell current Is₂when the second voltage (V2) is applied to calculate a subtracted valuesensitivity rate β that is a ratio between a value Is−I_(off), asderived by subtracting an offset current I_(off) that is a componentflowing regardless of presence of the given gas component and oxygenfrom the sensor cell current Is, and a value Im−I_(off), as derived bysubtracting the offset current I_(off) from the monitor cell current Im,using an equation below, and also determines the offset current I_(off)using an equation belowβ=(Is ₁ −Is ₂)/(Im ₁ −Im ₂)  (1)I _(off)=(Is ₁ −βIm ₁)/(1−β)  (2) and in that when measuring theconcentration of the given gas component in the gas, the calculatingportion measures the sensor cell current Is and the monitor cell currentIm and calculates the given gas dependent current Ix using an equationbelow to determine the concentration of the given gas component,Ix=Is−{β(Im−I _(off))+I _(off)}  (3).
 2. A gas concentration measuringapparatus as set forth in claim 1 characterized in that when the gascontains the given gas component, the calculating portion applies athird voltage (V3) and a fourth voltage (V4) which have values differentfrom each other to the pump cell and measures the monitor cell currentIm₃ and the sensor cell current Is₃ when the third voltage (V3) isapplied and the monitor cell current Im₄ and the sensor cell current Is₄when the fourth voltage (V4) is applied to calculate an approximatevalue β′ of the subtracted value sensitivity rate β using an equationbelow and also calculate an approximate value I_(off)′ of the offsetcurrent I_(off) using an equation belowβ′=(Is ₃ −Is ₄)/(Im ₃ −Im ₄)  (4)I _(off)′=(Is ₃ −β′Im ₃)/(1−β′)  (5) and in that when measuring theconcentration of the given gas component in the gas, the calculatingportion measures the sensor cell current Is and the monitor cell currentIm and calculates an approximate value Ix′ of the given gas dependentcurrent Ix using an equation below to approximately determine theconcentration of the given gas component,Ix′=Is−{β′(Im−I _(off)′)+I _(off)′}  (6).